What Does It Mean to 'Rehydrate' Brewed Coffee?

When you see someone adding water to coffee, you might often hear the term "By pass," which translates to "bypass" or "circumvent." At first glance, this seems unrelated to adding water to coffee. The reason this term appears here stems from our discussion of pour-over coffee brewing.

In the extraction process of pour-over coffee, hot water is used to extract coffee grounds, resulting in coffee liquid containing aromatic substances. This path can be considered normal, conventional extraction, which we can call the "main route." However, when your hot water bypasses the coffee grounds and directly falls to the bottom to mix with the coffee liquid, without participating in any extraction process, this behavior is called "bypass" or By pass.

What is the purpose of By pass?

FrontStreet Coffee often mentions: when coffee concentration is too high and flavors are overly concentrated, it can create confusion, making the coffee taste cluttered and difficult to distinguish specific flavors. At this point, we can use By pass to directly add water to dilute the high-concentration coffee. This allows flavors more space to develop, thereby achieving rich layers and enhanced experience.

Example

FrontStreet Coffee brewed a pot of Boundary Line Sidra using 20g of coffee grounds with 300ml of hot water, meaning a 1:15 coffee-to-water ratio. However, due to improper brewing, the resulting coffee only had intense sourness, with other flavors difficult to appreciate—present but not distinct.

At this point, we directly added about 20ml of 80°C hot water to the coffee liquid. Upon tasting again, we found very distinct grape, passion fruit, and citrus fruit flavors—all released and fully expressed. Additionally, the taste became cleaner and more transparent. Here are several points to note: first is the hot water temperature. Due to the principle of constant temperature, after coffee is brewed, the 92°C hot water has already cooled to about 80°C coffee liquid. Therefore, we don't use the original 92°C hot water but need approximately 80°C hot water for blending. Second, the amount of water added cannot be excessive; otherwise, it will cause over-dilution and reduce the coffee experience.

Other situations requiring By pass

1. Making iced coffee

Actually, making iced coffee falls under the use cases of By pass. For example, adding ice cubes to high-concentration cold brew or iced drip coffee is to dilute it, allowing flavors to be better released. Especially with iced pour-over, where part of the originally set extraction water volume is replaced with ice cubes, used to cool down and dilute the upcoming high-temperature, high-concentration coffee liquid.

2. "Rescuing" coffee that's about to fail

Since light roast coffee has a harder texture, hot water has difficulty penetrating for extraction. Therefore, we need finer grinding to increase the surface area of coffee grounds in contact with hot water, thereby improving extraction. However, fine grinding brings a certain amount of fine powder, which can easily cause blockage during brewing, thereby increasing the soaking time of coffee grounds. When soaking time is too long, it will extract negative flavors.

At this point, we can "cut off" the tail section, discarding the very end coffee liquid that takes forever to drip, and then taste. According to the perceived concentration strength, we plan the addition of small amounts of hot water to adjust the appropriate concentration, bidding farewell to a failed outcome.

3. Specially customized extraction methods

In large coffee competitions, we often see contestants using larger amounts of coffee grounds to extract high-concentration coffee liquid with positive flavors—that is, forcefully extracting the sweet and sour substances from the front section, then using By pass to dilute, reducing concentration while maintaining the original extraction rate. This actually shares similarities with the previously mentioned iced coffee making.

Situations where By pass cannot help

If you've already over-extracted and the brewed coffee has obvious bitterness and astringency, then in this situation, using By pass cannot reverse the outcome.

The core of By pass is dilution. When you use it on over-extracted coffee, it can only dilute the bitter coffee to make it less bitter. However, over-extraction flavors also come with a thin taste. After you additionally add hot water, it will turn from over-extracted coffee into "coffee-flavored water." Under-extracted coffee goes without saying!

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FrontStreet Coffee

No. 10, Bao'an Qianjie, Yandun Road, Dongshankou, Yuexiu District, Guangzhou, Guangdong Province

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